1. Field of Endeavor
The present invention relates to a method for assuring a durable (i.e., essentially during the complete operation interval) protection of thermal barrier coating systems and base metal parts of gas turbines and other heat engines, in particular from the deleterious effect of environmental contaminants present in the gas flow. In particular, the invention relates to a method of applying a protection on the ceramic surface and of renewing this protection regularly on-site.
2. Brief Description of the Related Art
Thermal barrier coatings (TBC) are commonly deposited onto parts of gas turbines and other heat engines in order to reduce the heat flow on the base metal. Materials such as Y-stabilized zirconia (YSZ) are frequently chosen for their intrinsically low thermal conductivity. An appropriate microstructure (i.e., porosity and pore geometry) can additionally enhance their insulating and strain tolerance properties (for example, as disclosed in an article in the Journal of the Ceramic Society 24 (2004) entitled “Modeling of thermal conductivity of porous material: Application to thick thermal barrier coatings”).
In the case of operation under extreme conditions (e.g., crude oil, heavy oil, presence of sand, sea water, etc.), porosity (and cracks) can be detrimental to the lifetime of the TBC system. Contaminants can infiltrate and diffuse into pores (and cracks), potentially inducing mechanical stresses and/or reaction with the TBC and/or with the bond coat (BC) and/or with the thermally grown oxide (TGO) layer. As a result, TBC spallation and/or bond coat corrosion may occur.
Consequently, a compromise has to be reached regarding the TBC microstructure, providing a balance between a highly open structure for an optimal thermal/mechanical management and a sufficient cyclic lifetime, and a dense or closed structure for a suitable protection against contaminants.
Environmental barrier coatings involving sealing, i.e., applying an impermeable layer onto the TBC system, are possible to protect the system against contaminants.
Different approaches have been followed so far:                Infiltration of the porosity of the TBC. Especially in the case of an APS (atmospheric plasma spraying) deposited layer, the horizontal fine pores are difficult to infiltrate. For wet processing, WO 2006/137890 proposes to immerse the substrate in a bath containing the solution and to subsequently apply vacuum in order to improve the infiltration.        Addition of one or several dense layer(s) on top of the TBC. A metallic layer in U.S. Pat. No. 5,169,674, composites in U.S. Pat. No. 5,851,678, or ceramics in WO-A-2001/83851, are for instance deposited on top of a TBC layer for such purpose.        Variation of the microstructure of the TBC layer as, e.g., disclosed in EP-A-1780308.        Remelting the uppermost layer of the TBC by laser glazing as, for example, in U.S. Pat. No. 6,933,061 or laser remelting as disclosed in U.S. Pat. Nos. 5,484,980 and 6,103,315.        
All approaches of the state-of-the-art are used off-site, i.e., are applied prior to mounting the protected parts and operating the machine, and they aim to prevent (or at least to render more difficult) the penetration of contaminants through the TBC layer by closing the surfacial open microstructure of the TBC.
Some of them claim that their system acts not only as a physical barrier but also as a reactive barrier against contaminants. The reactants (mainly involving alumina) react with corrosive species and increase as a result their melting point and/or their viscosity and prevent them from penetrating deeper into the TBC. Such so-called sacrificial oxide coatings are for instance described in U.S. Pat. Nos. 6,261,643, 5,660,885, and 5,773,141, and WO-A-96/31293.
Since sacrificial coatings are consumed due to reaction, their durability is clearly an issue. Under extreme conditions, such as for operation under crude or heavy oils with possible sand infiltration, erosion tremendously affects coatings. In general, all sealants mentioned above tend to have a reduced thermal cycling resistance and a reduced total lifetime mainly due to the decreased strain tolerance of the system. Thus, the benefit of sealing against contaminants is generally only temporary and insufficient to withstand one complete operation interval. In consequence, the state-of-the-art protections are degraded very fast and the available technologies have not proved to perform to expectations.